We have discovered that scattered light intensity fluctuations (SLIF) are present in isolated rat ventricular muscle even under conditions formerly considered to be quiescent. Subsequent experiments indicated that SLIF are highly dependent on calcium loading of the cell and could be reversibly terminated (1) by maintaining constant calcium concentration in the myofilament space in skinned fibers or (2) in intact fibers by caffeine. These results were interpreted to indicate that cellular myoplasmic calcium concentration oscillates in diastole, producing motion of the myofilaments, which modulates the laser beam resulting in SLIF. This myofilament motion which is asynchronous within a cell, and among cells, results in a small degree of diastolic force or "tone" in the muscle. We have also examined the relationship of the SLIF frequency and force development in response to subsequent excitation and have found very high correlations. Thus, the oscillatory calcium pool monitored by SLIF appears to be intimately related to excitation-contraction coupling in muscle. More recent experiments have demonstrated SLIF in atrial, ventricular and conduction tissues in a range of mammalian species including man and indicate the universality of this phenomenon in excitable cardiac tissues.